1. Field of the Invention
The present invention relates, generally, to methods and apparatus for sensing objects on a conveyor, and in particular, to methods and apparatus for sensing successive signatures of printed matter.
2. Description of the Prior Art
In various contexts it is necessary to sense, and maintain a count of, objects being conveyed past a given point. For example, in the printing industry, it is often necessary to sense and maintain a count of signatures passing various points in a printing or binding operation. Mechanisms for sensing signatures, often referred to as signature counters (which generate corresponding electrical pulses to a counter) are, in general, known. For example, prior art signature counters are described in U.S. Pat. No. 3,702,925 issued to Anderson et al. on Nov. 14, 1972 and, U.S. Pat. No. 4,562,339 issued to Sjogren et al. on Dec. 31, 1985.
Such prior art counters tend to be disadvantageous in that they do not readily accommodate objects of varying height or stacks of overlapping objects, such as, e.g., overlapping signatures. For example, certain prior art signature counters include a mechanical trip switch supported by respective rollers, with the arm of the trip switch extending radially beyond the perimeter of the rollers. Typically, the roller/switch assembly is mounted over the conveyor on a pivotal lever arm, or on guide posts, to permit the vertical disposition of the counter relative to the conveyor surface to vary, the rollers being offset from the conveyor surface and the trip arm extending downwardly and forwardly (opposite the direction of conveyance) to a predetermined level above the conveyor (proximate to the conveyor surface where relatively thin signatures are to be sensed). In operation, the leading edge of a signature trips the switch arm, and thereafter the rollers ride over the edge of the signature, lifting the unit to a higher vertical level. Such prior art counters tended to be relatively massive and the response time to variations in height (or numbers of overlapping objects) tended to be slow, placing limitations on the speed of the conveyor. In addition, where prior art signature counters were used with a high speed conveyor, contact between the counter and the signatures tended to cause bounce, often resulting in miscounts. Further, where the prior art counters were designed for high sensitivity, they tended to be particularly susceptible to spurious counts caused by in continuities in the surface of the object, such as bunching in the signature.
Accordingly, there is a need for a sensor, and particularly a signature counter, that can be mounted in a fixed position without necessitating adjustment for sensing objects such as signatures of various sizes or lapping of varying numbers of signatures, and which is of relatively high sensitivity yet tolerant of surface irregularities in the object, e.g., rippling (bunching) in a signature, and which reacts quickly to changes in the height of the objects being counted.